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 Nonlinear curve fit: parameters do not vary		  New Topic  Reply to Topic
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studentAgain

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Posted - 07/07/2014 :  5:20:20 PM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic
Origin Ver. and Service Release (Select Help-->About Origin):
Operating System: OriginPro 8.1 SR3, Windows 8.1

Hello,

I have a quick question about the nonlinear curve fitting. When I try to fit my data with for instance, Polynomial cubic function
y = A + B*x + C*x^2 + D*x^3
everything works great when I use automatic parameter initialization, however; as soon as I try to change slightly any of the parameters manually (i.e by typing its value in the parameter box) the fit becomes poor and does not converge any more. I click "fit until converged" button but nothing happens (it shows a message: "cannot find direction to change parameters to reduce Chi-sqr"), the parameter that I just manually entered appears to be frozen/fixed and does not change anymore. I do not understand why and would really appreciate any help.

Thank you,

Shirley_GZ

China
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Posted - 07/07/2014 :  11:22:55 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Hi,

I can't reproduce the problem, please send us sample data and tell us the details to help us reproduce the problem here.

Via: tech@originlab.com

Shirley

Originlab Technical Service Team
Edited by - Shirley_GZ on 07/07/2014 11:32:15 PM
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studentAgain

8 Posts
Posted - 07/10/2014 :  5:27:12 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Thank you for your reply. I've sent the data and more detailed description of the problem to the above address. Looking forward to your reply!
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